1. Field of the Invention:
This invention relates to improved foam materials, particularly for food service and food packaging applications. These foams are made from starshaped polymers with a plurality of poly(hydroxyacid) chains (polylactide, polyglycolide, polycaprolactone, etc.) attached to a central polyfunctional compound having a plurality of hydroxyl and/or amino groups.
The materials of this invention are degradable, both biodegradable and hydrolyzable. Furthermore, they are foamed with environmentally benign gas and so contain no stratosphere ozone-depleting chemicals nor chemicals that have significant low altitude smog forming photochemical ozone producing reactivity. They have little global warming potential compared to chlorofluorocarbon (CFC) blowing agents. Thus, this invention provides readily degradable foam packaging materials with minimal environmental impact.
2. Description of the Related Art:
Copending and commonly assigned U.S. patent application Ser. No. 07/826,357, allowed Mar. 3, 1992, relates to poly(hydroxy acid) foam materials. The polymers for the foams of the present invention are branched star polymers containing hydroxy acidic acid units polymerized with other units that greatly enhance their foaming capabilities and the properties of the resultant foams.
At the present time, the most commonly used food service and food packaging materials are light weight closed cell foamed polystyrene, usually foamed with chlorofluorocarbon (CFC) or hydrocarbon (HC) blowing agents. These products pose serious environmental problems.
First, polystyrene is not degradable, either in landfills or as roadside litter. Thus, unless collected and recycled, polystyrene foam products have an unlimited litter life.
Second, CFC blowing agents trapped in the foam products, which make up a high volume percentage of the products, eventually escape to the stratosphere where the chlorine enters an ozone-depletion cycle.
Third, if HC blowing agents are used in place of CFCs, upon release from the foam they are photochemically reactive and thus promote smog formation. Also, they are flammable.
Thus, there is a need for light weight closed cell foam of a degradable resin foamed by a blowing agent that does not enter into chemical reactions that cause environmental damage.
Degradable polylactides are shown in Murdoch U.S. Pat. No. 4,766,182 (Aug. 23, 1988) and 4,719,246 (Jan. 12, 1988). These patents also disclose forming open cell porous structures by the extraction of solvent from polylactide gel. These open cell structures cannot be used for containers for wet foods such as meats and drinks. Also, to the extent any of Murdoch's blowing agents are present in the porous product, they are not environmentally benign.
Battelle WO 90/01521 published Feb. 22, 1990 discloses degradable polylactide resins, and their processing into solid films. The films may be made into "foam" structures by dissolving therein a mixture of petroleum ether and methylene chloride, and then placing the film into boiling water to volatilize the liquid mixture. The Battelle "foam" films are irregular and unsuitable. Also, upon degradation these products will release a high volume percent of hydrocarbons that are deleterious to the atmosphere.
Zhu et al "Super Microcapsules (SMC). I. Preparation and Characterization of Star Polyethylene Oxide (PEO)-Polylactide (PLA) Copolymers.", Journal of Polymer Science: Part A: Polymer Chemistry, Vol 27, pages 2151-2159 (1989) describe polymerization of lactide from 3-arm and 4-arm hydrophilic poly (ethylene oxide) with hydroxylterminated arms.
Polylactic acid (PLA) polymers often show a very rapid drop in melt viscosity with increasing processing temperature. At temperatures which are high enough to allow processing of semicrystalline PLA polymers, the melt viscosity drops very fast (partly because of thermal degradation). As a result, a number of PLA melts have a low melt strength, which may affect the foam-forming capacity of these polymers. An ideal foam-forming PLA polymer should combine a lower processing temperature to limit thermal degradation during processing with a higher melt strength to improve the foam-forming capacity. Such combination of desirable features are approached with branched (star) PLA polymers used in the foams of the present invention.
A number of blowing agents, including fluorohydrocarbons, are known in the art. Spitzer U.S. Pat. No. 4,422,877 (Feb. 4, 1982) shows the use of a number of blowing agents, including 1,1-difluoroethane, to form foams from a number of resins. However, no PHA foam articles are disclosed.
Walter U.S. Pat. No. 4,988,740 (Jan. 29, 1991, filed Jun. 15, 1989) discloses closed cell elastic foam material made from elastomeric polymer. No rigid foam products are disclosed.
"The Elements of Expansion of Thermoplastics", Part I and Part II, James G. Burt, Journal of CELLULAR PLASTICS, May/June 1979 (Part I) and November/December 1978 (Part II) disclose in detail the mechanics of meltfoaming thermoplastic resins. This article, while not disclosing PHA foams, sets forth a large number of requirements that must be met by the foaming ingredients to prepare an acceptable foam product, such as: amount of plasticization of the molten resin by the blowing agent; volatility of the blowing agent at foaming temperature; speed of solidification of the blown resin on cooling; heat transfer necessary for solidification; molecular migration of the blowing agent through the blown cell walls; melt viscosity and melt strength of the resin during the cooling/solidification; the rate of change of polymer viscosity with temperature; and a number of other properties. One concludes that the suitability of particular polymeric resins to be foamed can be determined only by trial and error.
HFC manufacture is known in the art. See "Aliphatic Fluorine Compounds", A. M. Lovelace et al. (1958), p. 55.
The above patents, patent application and literature references are incorporated herein in total and made a part of this patent application.